Two-photon absorption, a type of multiphoton absorption, can be utilized for various applications featuring extremely high spatial resolution. As of now, available two-photon absorption compounds have low two-photon absorptivity. Thus, it is necessary to use an expensive, very high-power laser as a pumping source for inducing two-photon absorption.
Accordingly, to practically use a small, inexpensive laser for a technology utilizing two-photon absorption, it is necessary to develop highly efficient two-photon absorption compounds, and the sensitization technology thereof plays an important role.
Meanwhile, to sensitize one-photon absorption process based on optical principles, an enhanced surface plasmon field, which is excited on the surface of a metal, is used to optically measure a small amount of substance.
For example, a technology has been proposed in which an ultrathin film (an enhanced surface plasmon field is generated only at a region approximately 100 nm or less apart from the surface) is disposed on a thin metal film formed on a medium having a high index of refraction and is used as a sample for a surface plasmon microscope (e.g., refer to Patent Document 1).
Alternatively, a measurement technology has been known in which an enhanced surface plasmon field excited by fine metal particles is employed. Similar to the technology disclosed in Patent Document 1, an observation measurement region in this technology is limited to a region 100 nm or less apart from the periphery of the fine metal particle. Thus, highly sensitive observation can be performed by observing a sample adsorbed to the surface of the particle.
However, the technology disclosed in Patent Document 1 utilizes enhancement effects on the thin film, and a sample is limited (ultrathin film on the thin metal film).
Specifically, the region which can utilize enhancement effects of the surface plasmon depends on a shape of the thin metal film and an arrangement of an optical system. Therefore, it is difficult to apply the technology to applications such as three-dimensional process.
Moreover, to select a wavelength used for the observation, a technology has been known in which a resonant wavelength is tuned by a spherical core cell structure (e.g., refer to Patent Document 2).
The technology disclosed in Patent Document 2 utilizes an enhanced surface plasmon field generated around particles such as fine metal particles. Compared with the technology disclosed in Patent Document 1, the shape of a substance generating an enhanced field is less limited. However, the particle generating the enhanced surface plasmon field sensitizes one-photon absorption, and the application area is limited only to fine particles. Thus, the technology has disadvantages that a limited wavelength can be used and the application area is limited for practical use.
Furthermore, a highly sensitive observation method using aggregate nanoparticles disposed in microcavities has been proposed, including a multiphoton process (e.g., refer to Patent Document 3).
A technology disclosed in Patent Document 3 has a disadvantage that applications of the enhanced field are limited since the aggregate nanoparticle generating the enhanced surface plasmon field is disposed in a microcavity.
Meanwhile, instead of the above fine metal particle, a technology utilizing a gold nanorod has been studied to generate an enhanced surface plasmon field. Gold nanorods can change a resonant wavelength by changing its aspect ratio. The gold nanorod is a material compatible with light having a wavelength from approximately 530 nm to the near infrared region (approximately 1100 nm).
A method has been proposed in which the gold nanorod is prepared by electrochemical reaction in a solution containing a surfactant (e.g., refer to Patent Document 4).
In the technology disclosed in Patent Document 4, a range of an excitation wavelength is wider for an enhanced surface plasmon field generator capable of tuning a wavelength. However, an arrangement of a pumping source and a reactive substance are problematic.
The inventors of the present invention have proposed in advance that a multiphoton absorption material (e.g., a two-photon absorption material) is mixed with a fine metal particle or a gold nanorod to achieve two-photon sensitization, and the mixture is applied to a device (e.g., refer to Patent Document 5).
The technology disclosed in Patent Document 5 is excellent for sensitizing multiphoton absorption reaction. However, the technology has a disadvantage.
Specifically, fine metal particles and metal nanorods are dispersed in an aqueous solvent to be used. By contrast, most of the multiphoton absorption materials are difficult to be dissolved in water. Thus, to obtain a thin film or a bulk from the mixture of the multiphoton absorption material and the fine metal particle or the metal nanorod, it is necessary to uniformly disperse the fine particles or the rods in an organic solvent.
A two-photon absorption material, one of the multiphoton absorption materials, can excite molecules in a wavelength of non-resonant region. An actual excited state of the material is at an energy level approximately twice as much as that of photons used for the excitation.
Two-photon absorption, a type of multiphoton absorption, is a type of three-dimensional nonlinear optical effects. A molecule absorbs two photons simultaneously to transit from the ground state to the excited state. The two-photon absorption has been known for a long period of time. Study on a material having two-photon absorptivity has been recently advanced since Jean-LucBredas et al. unraveled the relationship between the molecular structure and mechanism in 1998 (Science, 281, 1653 (1998)).
However, the transition efficiency of this simultaneous two-photon absorption is much less than that of one-photon absorption, and the two-photon absorption requires photons having an extremely large power density. Accordingly, two-photon absorption is hardly observed with a normally used laser light intensity. When an ultrashort (femtosecond) pulse laser is used (e.g., a mode locking laser having a high peak light intensity (light intensity at the maximum luminous wavelength)), it is confirmed that two-photon absorption is observed.
The transition efficiency of the two-photon absorption is proportional to the square of the intensity of an applied optical field (square-law characteristics of two-photon absorption). Accordingly, when a laser beam is applied, two-photon absorption occurs only at a position where field intensity is high in the center of a laser spot, and two-photon absorption does not occur at a position where field intensity is low in the peripheral portion. In a three-dimensional space, two-photon absorption occurs only at an area where field intensity is high at a focal point of the laser beam condensed by a lens. Two-photon absorption does not occur at all an area outside the focal point where field intensity is low. Compared with one-photon linear absorption in which excitation occurs at all the positions proportionally to the intensity of the applied optical field, excitation occurs only at a pinpoint region in a space due to the square-law characteristics. Thus, spatial resolution is significantly improved.
A three-dimensional memory, which records bit data by utilizing the above characteristics to change spectrum, index of refraction, or polarized light due to two-photon absorption at a predetermined position of a recording medium, has been studied. As described above, two-photon absorption occurs proportionally to the square of the light intensity. Thus, the memory utilizing the two-photon absorption is enabled to perform ultra resolution recording since its spot size is smaller than that of the memory utilizing the one-photon absorption. In addition, the two-photon absorption has high spatial resolution due to the square-law characteristics. Therefore, applications of the two-photon absorption to optical power limiting materials, photocuring resins (a photocuring material) for stereolithography, fluorescent dye materials for a two-photon fluorescence microscope, and the like have been developed.
To induce two-photon absorption, it is possible to employ a short pulse laser of the near infrared region which has a longer wavelength than a wavelength region having a linear absorption band of the compound and in which no absorption occurs.
Since near infrared light of a transparent region, which does not have a linear absorption band of the compound, is employed, excitation light can reach, for example, the inside of a sample, without being absorbed or scattered. Besides, a pinpoint region in the sample can be excited with high spatial resolution due to the square characteristics of the two-photon absorption. Thus, two-photon absorption and two-photon luminescence are expected to be applied to two-photon contrast imaging of a biological tissue and photo-chemotherapy such as two-photon photodynamic therapy (PDT).
Moreover, when the two-photon absorption or two-photon luminescence is employed, photons having energy higher than that of incident photons can be taken out. Thus, study on up-conversion lasing has been reported in terms of a wavelength conversion device.
Various inorganic materials have been discovered as the two-photon absorption materials. However, it is difficult to design molecules in the inorganic material to obtain desired two-photon absorption characteristics and other necessary characteristics for element manufacturing.
By contrast, desired two-photon absorption can be obtained by designing molecules in an organic compound. Moreover, other characteristics can be controlled in the organic compound. The organic compound has a potential for practical use and is considered as a prospect two-photon absorption material.
Dye compounds such as rhodamine and coumalin, dithienothiophene derivatives, and oligophenylenevinylene derivatives are known as conventional organic two-photon absorption materials. However, these compounds have small two-photon absorption sectional areas per molecule, which indicate two-photon absorptivity. When a femtosecond pulse laser is used, most of the two-photon absorption sectional areas are less than 200 (GM: ×10−50 cm4·s·molecule−1·photon−1). Thus, industrial applications of these compounds have not been made yet.
For example, regarding a field to which a three-dimensional multilayered optical memory using a two-photon absorption material is expected to be applied, network such as the Internet and a high vision television are recently and rapidly spread. Moreover, high definition television (HDTV) broadcast is starting soon, and the demand for mass storage recording media has been increasing to record image information of 50 GB or more and preferably 100 GB or more inexpensively and simply for consumer use. Furthermore, optical recording media have been demanded for business applications such as computer and broadcast backups to record mass information of approximately 1 TB or more inexpensively at high speed.
Meanwhile, the capacity of a conventional two-dimensional optical recording medium such as DVD±R is about 25 GB although a recording/reproducing wavelength is shortened based on physical principles. This capacity is not enough for the future use.
In this present situation, a three-dimensional optical recording medium has been prominently expected as an ultimate high density, high capacity recording medium. In the three-dimensional optical recording medium, recordings can be superposed on several tens or several hundreds of layers in a three-dimensional (thickness) direction. Alternatively, it is possible to perform recording/reproducing repeatedly in a light incident direction when the recording layer is considered as a thick film. By using the three-dimensional recording medium, ultra high density, ultra high capacity recording can be achieved, which is several tens or several hundreds higher than the conventional two-dimensional recording medium.
To provide a three-dimensional optical recording medium, information needs to be written with access to a predetermined area in a three-dimensional (thickness) direction. To achieve this, a two-photon absorption material or holography (interference) may be used.
In the three-dimensional optical recording medium using a two-photon absorption material, bit recording which can record information several ten or several hundred times as much as the conventional recording, is enabled based on the aforementioned physical principles, thereby enabling higher density recording. Therefore, the three-dimensional optical recording medium is an ultimate high density, high capacity optical recording medium.
For example, for the three-dimensional optical recording medium using the two-photon absorption material, a method for reading with fluorescence by using a fluorescent substance for recording/reproducing (e.g., refer to Patent Documents 6 and 7), a method for reading with absorption or fluorescence by using a photochromic compound (e.g., refer to Patent Documents 8 and 9), and the like have been proposed.
However, Patent Documents 6 and 7 do not describe a specific two-photon absorption material, but abstractly describes a two-photon absorption compound such as a two-photon absorption compound with extremely small two-photon absorption efficiency, for example.
Moreover, the photochromic compounds used in Patent Documents 8 and 9 are reversible materials, there are problems of nondestructive readout, long-term archivability of recording, an S/N ratio of reproducing, and the like. Thus, the photochromic compounds are not suitable for an optical recording medium in practical use. In terms of nondestructive readout, long-term archivability of recording and the like, an irreversible material is preferably used to change reflectivity (index of refraction and absorptivity) or luminescence intensity to reproduce. However, there is no specific description on the two-photon absorption material having these properties.
A recording apparatus, a reproducing apparatus, readout, and the like have been proposed for three-dimensional recording by changing the index of refraction (e.g., refer to Patent Documents 10 and 11). However, there is no description on a method for using two-photon absorption three-dimensional optical recording medium.
It is known that fine (spherical) silver particles and a styrylbenzene two-photon material are linked by an alkyl chain to exert enhancement effects by two-photon characteristics (e.g., refer to Non-Patent Document 1). In this case, light having a wavelength which can efficiently exert two-photon absorption characteristics the most (i.e., 740 nm to 900 nm) does not resonate with (is not absorbed to) fine (spherical) silver particles. Thus, it is impossible to enhance the effects of the enhanced surface plasmon field.
[Patent Document 1] Japanese Patent Application Laid-Open (JP-A) No. 2004-156911
[Patent Document 2] JP-A (Translation of PCT Application) No. 2001-513198
[Patent Document 3] JP-A (Translation of PCT Application) No. 2004-530867
[Patent Document 4] JP-A No. 2005-68447
[Patent Document 5] JP-A No. 2006-330683
[Patent Document 6] JP-A (Translation of PCT Application) No. 2001-524245
[Patent Document 7] JP-A (Translation of PCT Application) No. 2000-512061
[Patent Document 8] JP-A (Translation of PCT Application) No. 2001-522119
[Patent Document 9] JP-A (Translation of PCT Application) No. 2001-508221
[Patent Document 10] JP-A No. 6-28672
[Patent Document 11] JP-A No. 6-118306
[Non-Patent Document 1] Wim Wenseleers et al., J. Phys. Chem. B 2002, 106, 6853-6863